The primary goal of this project is to understand how microsporidia cause disease. Microsporidia are eukaryotic intracellular pathogens that can infect a wide variety of hosts but are poorly understood. These pathogens can cause serious infections in humans, particularly in AIDS patients. Understanding the pathogenic properties of microsporidia is relevant to the NIH's mission because it promises to help reduce the burden of human illness. Very little is known about the molecular interactions between microsporidia and their host intestinal cells, although several observations suggest that there is an intimate dynamic between them. Our long-term goal is to better understand the strategies that microsporidia use to exploit host intestinal cells, as well as the strategies hosts use to control these infections. The objective of this proposal is to investigate the molecular interactions between the nematode C. elegans and its natural microsporidian pathogen, N. parisii. N. parisii infects C. elegans intestinal cells, which are transparent, facilitating direct imaging of the infection. C. elegans intestinal cells share many features with mammalian intestinal cells, such as finger-like microvilli, which are anchored into a cytoskeletal structure called the terminal web. Intriguingly, N. parisii can specifically manipulate the terminal web, probably as part of an exit strategy. The central hypothesis of this proposal is that there are specific molecular interactions between host and pathogen that allow N. parisii to directly target host cell components, and that allow C. elegans to regulate resistance to infection. We will pursue two specific aims to address this hypothesis: 1) determine the mechanisms used by N. parisii to restructure and modify C. elegans intestinal cells; and 2) identify the pathways that are important for regulating resistance/susceptibility to N. parisii infection. To accomplish the first aim we will use in vivo imaging of N. parisii infection in animals expressing fluorescently-labeled cytoskeletal and subcellular markers in the C. elegans intestine. These studies will be complemented with physiological and biochemical analyses of the consequences of infection. Under the second aim we will use a combination of genetic and genomic techniques to identify which pathways are important for control of N. parisii infection. The approach is innovative because it utilizes a natural host/pathogen model that offers genetic, molecular, and imaging tools to better understand molecular interactions between microsporidia and their hosts. The proposed research is significant because it is expected to provide insights into the molecular interactions between microsporidia and their human hosts.